The present invention relates to a slitting unit forming part of a machine for production of carton blanks, in particular blanks made of cardboard or corrugated cardboard, which unit comprises working wheels which are opposite in pairs, one wheel in at least one pair being made with at least one slitting knife extending over a part of the periphery of the wheel in said pair while the periphery of the other wheel is made with a slot for receiving the knife, and also members for feeding plane carton blanks between the wheels.
Slitting units of this type are used for providing carton blanks with slits which define foldable flaps which are required for the subsequent erection of the blank to form a carton. In this connection, the unit usually comprises a number of slitting-wheel pairs arranged one after the other and beside one another in order to produce both slits which are open towards the front and, respectively, rear edges of the blanks and slits which are separated in the lateral direction.
In conventional slitting units, the blanks are fed by means of the slitting wheels provided with knives and corresponding stay wheels made with pads, which requires the blanks to be clamped between the wheels.
In order to achieve the necessary friction against the blanks so that these are guided accurately, the blanks will be pressed together and deformed slightly by the slitting wheels. Furthermore, text and images which have been printed on the blank and have not dried may be smeared. In spite of strong contact against the blanks, there is still a risk of these being pulled through the unit at an angle.
In order to achieve the desired pressure against the carton blank, the slitting wheels in each pair must be height-adjusted depending on the thickness of the carton blank concerned, which is time-consuming and awkward. When the slitting wheels are used as feed wheels, their axes must be centred along the same vertical line, which means that the point where a knife cuts through the blank is located in the air in front of the point, the cutting point, where the knife meets the stay wheel. The distance to the cutting point increases as the blank thickness decreases.
As the slitting wheels are used as feed wheels, the smallest possible blank thickness is determined by the diameter of the wheels. As it is often desirable to combine slitting wheels with creasing wheels, a problem arises. For the creasing wheel, it is desirable to use as great a diameter as possible so as to reduce the velocity component in relation to the blank for points on the periphery of the creasing wheel in order thus to reduce the risk of obtaining a cutting effect instead of creasing. This results in the machine not being capable of handling small blanks as the wheels must then be positioned with a relatively great distance between axes seen in the direction of feed of the blanks.
The main object of the present invention is to produce a slitting unit in which inter alia the abovementioned disadvantages are eliminated.
In machines for production and handling of carton blanks, it is previously known to fix and feed the blanks in some handling stations by means of vacuum mats. This has previously not been possible in slitting units because inter alia the problem has not been solved of how the slitting knives are to be capable of cutting through a blank if it is located on a vacuum mat.
The present invention is based on the insight that it is possible to use vacuum technology for fixing and transporting carton blanks even in a slitting unit, provided that the vacuum mat is divided into a number of separate vacuum belts which are each supported by an associated stand unit.
In this connection, a slitting unit of the type indicated in the first paragraph is, according to the invention, characterized in particular in that it comprises a vacuum belt running adjacently to each slitting-wheel pair for securing and feeding the blanks, in that the wheels in each slitting-wheel pair are mounted in an upper and, respectively, a lower stand unit, in that at least a portion of that part of one stand unit which faces towards the other is made in the form of a vacuum box, and in that the vacuum belt runs around said one stand unit and over the vacuum box, it being possible for the blanks to be held fixed on the belt by the effect of suction from the vacuum box during transport through the slitting-wheel pair.
In a slitting unit of this type, no clamping of the carton blank between the slitting wheels is required, which means that a number of the abovementioned problems are eliminated. Nor is it necessary to perform any adjustment of the mutual distance of the slitting wheels in the vertical direction depending on the thickness of the carton blank concerned. Moreover, the slitting wheels in each pair can be displaced slightly relative to one another in the direction of feed of the blanks, which means that the cutting point can be adjusted so that, irrespective of the thickness of the blank concerned, it is located essentially in the lower delimiting surface of the blanks.
It is preferred that the slitting unit comprises a number of stand units arranged beside one another, with slitting-wheel pairs and vacuum belts, and that all the vacuum belts are driven synchronously with one another so as to form a common conveyor for the blanks. The stand units with their associated slitting wheels are suitably individually displaceable in a direction at right angles to the direction of feed of the blanks in order to allow adjustment of the positions of the slits depending on the size of the blanks.
Further features of a slitting unit according to the invention emerge from the patent claims below.